1. Field of the Invention
The present invention relates generally to a valve timing control apparatus that includes a timing change mechanism for controlling the valve timing of the intake valve or exhaust valve of an engine. More particularly, this invention relates to a valve timing control apparatus that continuously controls the valve timing in accordance with the running conditions of the engine.
2. Description of the Related Art
In the conventional engine with an ordinary structure, the intake valve and exhaust valve operate to selectively open or close respective air-intake and exhaust passages, which are connected to the individual combustion chambers. Those valve timings are specifically synchronous with the rotational phase of the crank shaft and thus the timing of the up-and-down movement of the pistons. Therefore, the intake amount into any one of the combustion chambers and the exhaust amount therefrom vary depending on the angle of a throttle valve provided separately in the engine's air-intake passage, or on the speed of the engine.
Various apparatuses are available that alter the valve timing in order to control the intake and exhaust amounts in the combustion chamber with a greater degree of freedom. Such apparatuses include a variable valve timing mechanism for changing the valve timing and a computer for controlling the operation of the variable valve timing mechanism. This computer controls the variable valve timing mechanism in accordance with the running condition of the engine to control the valve timing of the intake valve or the exhaust valve, thereby controlling the degree of valve overlapping of the intake valve and exhaust valve. Accordingly, the amount of the mixture of air and fuel to be supplied to each combustion chamber is controlled to optimize the air-fuel ratio of that mixture so that the engine power and emissions are properly adjusted.
When the engine speed is relatively high, for example, the computer controls the variable valve timing mechanism so as to relatively increase the valve overlapping. Under this control, the efficiency of the supply of air into the combustion chambers is enhanced by utilizing the inertia of the air that passes through the air-intake passage, thus improving the engine power. When the engine speed is relatively low, on the other hand, the computer controls the variable valve timing mechanism so as to relatively decrease the valve overlapping. This control prevents the exhaust gas, once discharged from the combustion chambers, from flowing back to the combustion chambers so that the exhaust gas remaining in the combustion chambers or the ratio of the internal EGR is reduced to prevent the miscombustion of the air-fuel mixture.
Japanese Unexamined Patent Publication No. Hei 4-279705 discloses one example of such a valve timing control apparatus. This apparatus is capable of changing the valve timing continuously and to the desired level in accordance with the running condition of the engine. As shown in FIG. 13, this apparatus has a cam position sensor 91 to detect the rotational position of a cam shaft 92. A crank position sensor 93 detects the rotational position of a crank shaft 94. First and second hydraulic pumps 95 and 96 pump out the oil of an oil pan 97. A variable valve timing mechanism 98, provided at the cam shaft 92, is hydraulically driven to change the rotational phase of the cam shaft 92. This mechanism 98 has a timing pulley 98a and incorporates a ring-shaped piston and a transmission member (both unillustrated), which couple the pulley 98a to the cam shaft 92. As the piston is moved by the hydraulic pressure, the rotational phase of the cam shaft 92 is changed. A hydraulic line 99 connects the second hydraulic pump 96 to the mechanism 98. First and second oil control valves (OCVs) 100 and 101 provided midway in the hydraulic line 99, control the supply of the hydraulic pressure to the mechanism 98. An electronic control unit (ECU) 102 computes the rotational phase of the cam shaft 92, or the target phase associated with the control of the valve timing, based on the value of the rotational speed of the crank shaft 94 (engine speed). The ECU 102 detects the actual phase of the valve timing based on the output signals of both sensors 91 and 93. The ECU 102 compares the detected actual phase with the computed target phase to compute a change value to be used in altering the rotational phase of the cam shaft 92. Based on this computed change value, the ECU 102 performs duty control of the opening s of both OCVs 100 and 101. Accordingly, the mechanism 98 is controlled to provide the optimal valve timing in accordance with the running condition of the engine 103.
To advance the valve timing from the current timing, for instance, the ECU 102 fully closes the second OCV 101 and executes the duty control on the opening of the first OCV 100 in accordance with the aforementioned change value. When the value of a change in the rotational phase of the cam shaft 92 matches with the target value, the ECU 102 fully closes both OCVs 100 and 101 to sustain the valve timing. This control allows the hydraulic line 99 to be tightly closed and maintains the value of a change in the rotational phase of the cam shaft 92, so that the valve timing of that instant is sustained. When the oil leaks from somewhere in the hydraulic line 99, the valve timing may vary. In this respect, the ECU 102 controls both OCVs 100 and 101 while always detecting the valve timing to execute feedback control of the valve timing. To retard the valve timing from the current timing, the ECU 102 fully closes the first OCV 100 and executes duty control on the opening of the second OCV 101 in accordance with the change value. When the value of a change in the rotational phase of the cam shaft 92 matches with the target value, the ECU 102 likewise fully closes both OCVs 100 and 101 to retard the valve timing. This control maintains the value of a change in the rotational phase of the cam shaft 92, so that the valve timing of that instant is sustained.
In this valve-timing sustaining control to sustain the valve timing to the target phase, the apparatus disclosed in the aforementioned publication performs no control to evaluate the control result and to learn a learning value for correcting the valve-timing sustaining control based on the evaluation result. In other words, this prior art lacks optimal learning control in the case where learning control is adapted to the sustaining control.
The output characteristics of the mechanism 98, both OCVs 100 and 101, vary depending on their allowances or their time-dependent changes. The output characteristics also vary depending on the running condition of the engine 103. As the rotational speed of the engine 103 or the warm-up state thereof varies, the levels of the hydraulic pressures acquired by both pumps 95 and 96 differ from each other. This difference in hydraulic pressure causes the output characteristics of the individual members 98, 100, 101, etc. to vary. To eliminate the influence of the allowances or time-dependent changes of the individual members 98, 100, 101, etc. from the sustaining control, therefore, learning control should be adapted to the sustaining control. For adaptation of the learning control to the sustaining control, there are several problems.
For example, one of the problems is to determine what should be the initial value to be used in the initial learning process in consideration of the allowances or time-dependent changes of the individual members 98, 100, 101, etc. When the initial value is improper, the sustaining control may bring about the improper result during the period in which the initial learning is completed temporarily. Another problem is to determine on what should be the learning value used in the sustaining control when the mechanism 98 recovers from the failure-originated fixed state on the premise that the valve-timing sustaining control should be executed. When an improper learning value is set, the sustaining control may bring about an improper result during the period in which the learning is completed temporarily after the recovery from the failure. If the adjustment of either value is inadequate, the proper valve timing cannot be acquired temporarily. If the valve timing is too advanced temporarily, the valve overlapping becomes too large temporarily and the combustion of the air-fuel mixture in the combustion chambers becomes unstable, which may result in misfire or engine stalling.
In addition, the mechanism 98 may temporarily become inoperable due to mechanical restriction. Such may occur when the piston in the mechanism 98 moves and comes to its end position. In this case, with simple execution of the sustaining control, it is not possible to distinguish the stop of the mechanism 98 by the sustaining control from the stop of the mechanism 98 by the mechanical restriction thereof. When learning control is adapted to the sustaining control, therefore, an inaccurate learning value may be obtained in the learning control. Further, even if the learning value is updated in the learning control, the actual phase of the valve timing may not converge to the target phase.